1. Field of the Invention
The invention relates to methods and apparatus for accessing the ability of a vehicle emissions control device, such as a lean NO.sub.x trap, to releasably store an exhaust gas constituent.
2. Background Art
The exhaust gas generated by a typical internal combustion engine, as may be found in motor vehicles, includes a variety of constituent gases, including hydrocarbons (HC), carbon monoxide (CO), nitrogen oxides (NO.sub.x) and oxygen (O.sub.2). The respective rates at which an engine generates these constituent gases are typically dependent upon a variety of factors, including such operating parameters as air-fuel ratio (8), engine speed and load, engine temperature, ambient humidity, ignition timing ("spark"), and percentage exhaust gas recirculation ("EGR"). The prior art often maps values for instantaneous engine-generated or "feedgas" constituents, such as HC, CO and NO.sub.x, based, for example, on detected values for instantaneous engine speed and engine load (the latter often being inferred, for example, from intake manifold pressure).
To limit the amount of feedgas constituents that are exhausted through the vehicle's tailpipe to the atmosphere as "emissions," motor vehicles typically include an exhaust purification system having an upstream and a downstream three-way catalyst. The downstream three-way catalyst is often referred to as a NO.sub.x "trap". Both the upstream and downstream catalyst store NO.sub.x when the exhaust gases are "lean" of stoichiometry and releases previously-stored NO.sub.x for reduction to harmless gases when the exhaust gases are "rich" of stoichiometry.
More specifically, in a typical embodiment, the trap chemically stores NO.sub.x during lean-burn operation using alkaline metals, such as barium and/or strontium, in the form of a washcoat. The NO.sub.x (NO and NO.sub.2) are stored in the trap in the form of barium nitrate, for example. The washcoat also includes precious metals, such as platinum and palladium, which operate to convert NO to NO.sub.2 for storage in the trap as a nitrate. The trap's washcoat typically also includes ceria, whose affinity for oxygen storage is such that, during initial lean engine operation, a quantity of the excess oxygen flowing through the trap is immediately stored in the trap. The amount of stored oxygen is essentially fixed, although it begins to lessen over time due to such factors as increased trap sulfurization (sulfur accumulation) and trap aging.
The trap's actual capacity to store NO.sub.x is finite and, hence, in order to maintain low tailpipe NO.sub.x emissions when running "lean," the trap must be periodically cleansed or "purged" of stored NO.sub.x. U.S. Pat. No. 5,473,887 teaches the purging of a NO.sub.x trap by subjecting the trap to an air-fuel mixture whose air-fuel ratio is rich of stoichiometric, for example, an air-fuel ratio of less than about 13. During the purge event, excess feedgas HC and CO, which are initially consumed in the three-way catalyst to release stored oxygen, ultimately "break through" the three-way catalyst and enter the trap, whereupon the trap's barium nitrate decomposes into NO.sub.2 for subsequent conversion by the trap's precious metals into harmless N.sub.2 and O.sub.2. The oxygen previously stored in the trap is also released during an initial portion of the purge event after the HC and CO break-through the three-way catalyst.
Because a finite amount of excess fuel is required during the trap purge to release the stored oxygen before stored NO.sub.x is released, the prior art has sought to estimate the amount of oxygen stored in the trap by inferring oxygen storage from the time period between combustion, immediately after a rich purge event, of a lean air-fuel mixture within the engine and the switching of the downstream air-fuel ratio from a near-stoichiometric air-fuel ratio to a lean air-fuel ratio, as measured by a downstream oxygen sensor. However, because a significant portion of the time period between the commencement of lean engine operation and the switching of the downstream sensor is caused by movement of the resulting lean exhaust gas through the vehicle's exhaust system, both upstream and downstream of the trap, the prior art teaches an involved process for separating out the resulting time lags from the overall period, in an attempt to obtain an accurate measure of oxygen storage. Changes in the engine operating condition, i.e., the engine's speed and load, during this period renders the determination of the trap's oxygen storage capacity increasingly problematic.
Therefore, a need exists for a method and apparatus for accessing the ability of an emissions control device, such as a lean NO.sub.x trap, to releasably store an exhaust gas constituent.